RESUMO
The complement system is an essential component of the innate and acquired immune system, and consists of a series of proteolytic cascades that are initiated by the presence of microorganisms. In health, activation of complement is precisely controlled through membrane-bound and soluble plasma-regulatory proteins including complement factor H (fH; ref. 2), a 155 kDa protein composed of 20 domains (termed complement control protein repeats). Many pathogens have evolved the ability to avoid immune-killing by recruiting host complement regulators and several pathogens have adapted to avoid complement-mediated killing by sequestering fH to their surface. Here we present the structure of a complement regulator in complex with its pathogen surface-protein ligand. This reveals how the important human pathogen Neisseria meningitidis subverts immune responses by mimicking the host, using protein instead of charged-carbohydrate chemistry to recruit the host complement regulator, fH. The structure also indicates the molecular basis of the host-specificity of the interaction between fH and the meningococcus, and informs attempts to develop novel therapeutics and vaccines.
Assuntos
Antígenos de Bactérias/química , Antígenos de Bactérias/metabolismo , Proteínas de Bactérias/química , Proteínas de Bactérias/metabolismo , Carboidratos/química , Fator H do Complemento/química , Fator H do Complemento/metabolismo , Mimetismo Molecular , Neisseria meningitidis/metabolismo , Sítios de Ligação , Fator H do Complemento/imunologia , Cristalografia por Raios X , Ligantes , Modelos Moleculares , Neisseria meningitidis/química , Neisseria meningitidis/imunologia , Ressonância Magnética Nuclear Biomolecular , Ligação Proteica , Conformação Proteica , Relação Estrutura-Atividade , Especificidade por SubstratoRESUMO
Nearly 50 million people worldwide suffer from age-related macular degeneration (AMD), which causes severe loss of central vision. A single-nucleotide polymorphism in the gene for the complement regulator factor H (FH), which causes a Tyr-to-His substitution at position 402, is linked to approximately 50% of attributable risks for AMD. We present the crystal structure of the region of FH containing the polymorphic amino acid His402 in complex with an analogue of the glycosaminoglycans (GAGs) that localize the complement regulator on the cell surface. The structure demonstrates direct coordination of ligand by the disease-associated polymorphic residue, providing a molecular explanation of the genetic observation. This glycan-binding site occupies the center of an extended interaction groove on the regulator's surface, implying multivalent binding of sulfated GAGs. This finding is confirmed by structure-based site-directed mutagenesis, nuclear magnetic resonance-monitored binding experiments performed for both H402 and Y402 variants with this and another model GAG, and analysis of an extended GAG-FH complex.
Assuntos
Envelhecimento/fisiologia , Fator H do Complemento/química , Fator H do Complemento/metabolismo , Sítios de Ligação , Fator H do Complemento/genética , Cristalografia por Raios X , Produtos do Gene gag/química , Produtos do Gene gag/genética , Produtos do Gene gag/metabolismo , Ligantes , Modelos Moleculares , Mutação/genética , Polissacarídeos/química , Polissacarídeos/metabolismo , Estrutura Quaternária de Proteína , Estrutura Terciária de Proteína , Sacarose/análogos & derivados , Sacarose/química , Sacarose/metabolismo , Propriedades de SuperfícieRESUMO
Human plasma protein complement factor H (FH) is an inhibitor of the spontaneously activated alternative complement pathway. An allotypic variant of FH, 402His, has been associated with age-related macular degeneration, the leading cause of blindness in the elderly. Crystals of FH domains 6-8 (FH678) containing 402His have been grown in the presence of a polyanionic sucrose octasulfate ligand (an analogue of the natural glycosaminoglycan ligands of FH) using both native and selenomethionine-derivatized protein. Native data sets diffracting to 2.3 A and SeMet data sets of up to 2.8 A resolution have been collected. An anomalous difference Patterson map reveals self- and cross-peaks from two incorporated Se atoms. The corresponding selenium substructure has been solved.